Voltage regulated rectifying system



Dec. 24, 1946. J. A. POTTER 2,413,033

VOLTAGE REGULA'IED RECTIFYING SYSTEM Filed July 29, 1942 2 Sheets-Sheet1 FIG. THERMIJ m 9 /o 13 I ,6

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INVENTOP J A. P0 TTEP ATTORNEY 1946. J. A. POTTER VOLTAGE REGULATEDRECTIFYING SYSTEM Filed July 29, 1942 2 Sheets-Sheet 2 FIG. 5

LOAD

% ERMISTOR OPERATING RAN 1 FIG. 6 l I g '1 1 i I I I i 1 NVENTOR J A.POTTER ATTORNEV Patented Dec. 24, 1946 2,413,033 YOLTAGE REGULATEDRECTIFYING SYSTEM James A. Potter, Rutherford, N. J., assignor to BellTelephone Laboratories, Incorporated, New York, N. Y., a corporation ofNew York Application July 29, 1942, Serial No. 452,728

Claims. 1

This invention relates to voltage regulators for alternating currentrectifying systems.

An object of the invention is to provide an inexpensive device forregulating the voltage of rectified current.

An additional object of the invention is to cause the voltage suppliedby a rectifying system to a load to be held substantially constantirrespective of variations in the load current. 4

Another object of the invention is to enable a thermistor employed in avoltage regulating systems to be placed at a sensitive point in itsresistance thermal characteristic under the condition of no load.

In accordance with the invention there is interposed between analternating current source of a rectifying system and the load circuitto which the rectifying system supplies unidirectional current a seriesimpedance element the resistance of which is a function of thetemperature of the element. When the element is cold its resistance isvery high. When its temperature rises the resistance decreases and at acertain point in its characteristic the rate of fall of the resistanceis extremely high. Such elements, known as thermistors, are described byPearson in the Bell Laboratories Record for December 1940. In theoperation of the rectifier system the resistance of the thermistor atthe low temperatures associated with low load current is relativelyhigh. As the load current rises the heating effect of the current uponthe thermistor also rises reducing the thermistor resistance tocompensate for the increased resistance of the rectifier and currentsupply circuit. Additional heating of the thermistor to accentuate thechange in its resistance may be obtained by associating it inphysicallyclose relation to the rectifier or the current supply transformer tofacilitate heat transfer to the thermistor. The thermistor may beinitially brought to the proper point on its resistance temperaturecharacteristic at no load by a separate heating circuit connected to anauxiliary winding of the current supply transformer. In order to preventthe introduction of ripples by the auxiliary heating circuit in the loadcurrent the heating circuit may include an auxiliary rectifier andfilter or the thermistor may be connected between the load currentrectifier and the load current filter.

Since the function of the thermistor is to reduce the voltage derivedfrom the alternating current source by an amount which varies in inversemanner with the load current, the thermistor may be placed either in analternating current input lead to the main rectifier or in a directcurrent output lead of that rectifier. The heat supplied to thethermistor by the auxiliary heating circuit may be controlled by a relayresponsive to the rectified voltage impressed upon the unidirectionalcurrent load circuit in such a manner as to close the auxiliary circuitwhen that voltage is low and to open it when it is high. The relay maythus supply intermittent heating effects to the thermistor which aresmoothed out by the thermal inertia of the thermistor.

Other features and aspects of the invention will be apparent from aconsideration of the following detailed specification taken inconnection with the accompanying drawings in which:

Fig. 1 is a circuit diagram of an alternating current rectifying systemwith provisions for regulating the voltage supplied to the load inaccordance with certain principles of this invention;

Fig. 2 is a circuit diagram of a modification of the system of Fig. 1 inwhich the series thermistor element is connected in the alternatingcurrent input circuit of the rectifier;

Fig. 3 is another modification in which the auxiliary heating circuit ofthe thermistor is provided with an auxiliary rectifier;

Fig. 4 illustrates a modification of the circuit of Fig. 3 in which thethermistor is connected between the current supply filter and the load;

Fig. 5 is a circuit diagram of a voltage regulated rectifying currentsupply system in which the series thermistor is provided with anadditional indirect heating circuit;

Fig. 6 is a graph employed to explain the operation of the circuit ofFig. 5; and

Fig. 7 is a circuit diagram of a system for regulating th voltage ofrectified current in which the indirect heating circuit of theregulating thermistor is intermittently energized in response to theoperation of a Wheatstone bridge connected directly across the loadcircuit.

Referring to Fig. 1 a source i of alternating current to be rectified isshown connected by a current supply transformer 2 to a full waverectifier 3 of well-known type the output leads 4 and 5 of which conductrectified current to the unidirectional current load 6 through asmoothing filter I. The transformer 2 is provided with an auxiliarysecondary winding 9 having a variable tap l0 and variable resistor I lin series therewith for supplying an alternating heating currentdirectly to thermistor 8. At the no load condition the only current inthe thermistor 8 is alternati g Cu rent derived from the secondarywinding 9 and the magnitude of which is determined by the position ofvariable tap l and the setting of variable resistor I l in order to fixthe temperature of thermistor 8 at a point for which the resistance ofthe thermistor lies within the required operating range. This is highlydesirable for the reason that the resistance temperature characteristicof the thermistor begins with a rapidly rising portion reaching a pointbeyond which the resistance rapidly falls and it is desired to operateon the falling portion of th characteristic. At higher loads there is apotential drop in the current supply transformer, the rectifier and thecircuit conductors which tends to reduce the rectified voltage availableat the load input terminals. Since, however, the temperature of thethermistor 8 rises rapidly with increased load current, the resistanceof the thermistor consequently undergoes a large decrease, thus reducingthe potential drop across the thermistor so that the unidirectionalvoltage applied to the load remains substantially constant. The initialsetting of the transformer tap and of the variable resistance may bechanged from time to time to take care of aging in the rectifier and toallow for operation in several ambient temperature ranges.

The filter i not only smooths out the rectified current but also keepsthe alternating current impressed directly upon the thermistor fromreaching the load. The elements of the full wave rectifier may be of thewell-known copper oxide type.

In order to still further accentuate the resistance change of thermistor8 with varying load the thermistor maybe mounted in close proximity tothe rectifier 3 to be subjected to the heat developed by the rectifier.If desired the rectifier 3 and thermistor 8 may be enclosed in a commonthermally insulated enclosure or housing [2 to stabilize the thermalenvironment and reduce extraneous effects.

Fig.2 discloses a modification of the system of Fig. 1 in which thethermistor i3 is connected in series in one of the alternating currentinput leads to the rectifier 3. In this circuit the thermistor l3 andthe transformer 2 may be mounted in close proximity to each other tofacilitate heat transfer from the transformer to the thermistor. Ifdesired they may be enclosed within a thermally insulated container orhousing i l.

The system of Fig. 3 is similar to that of Fig. 1 but adds an auxiliaryfull wave rectifier l5 in the circuit connecting transformer winding 9and the thermistor l6. Accordingly the heating current for thethermistor introduces into the output circuit of the main rectifier 3only a unidirectional voltage accompanied by ripples which may bereadily suppressed by the smoothing filter i.

In the system of Fig. 4 the thermistor Ill is supplied withunidirectional heating current over a path including the secondarywinding 9 of the current supply transformer, auxiliary full waverectifier l5, and an auxiliary smoothing filter Hi. It is accordinglypossible to connect, the thermistor l'i immediately adjacent the inputterminals of the load 6 since the thermistor is not subjected to heatingcurrent variations or ripples.

The system of Fig. 5 adds to the circuit of Fig. 1 a shunt pathconnected directly across the input terminals [9 and 2D to the load.This shunt path, as will be later explained in detail, is of suchcharacter as to draw a current IT which changes enormously for smallvariations in the electromotive force applied to the load inputterminals. Because of this very great change in the current mistor 22.

'over a range of In to Im.

IT through the shunt path two additional regulating effects are broughtabout. The first is occasioned by the fact that the shunt current ITtraverses the series thermistor 22 and therefore changes the resistanceof thermistor 22 and the consequent potential drop across the thermistoras does a change in the load current itself. In the second place thecurrent T1 of the shunt path is caused to traverse an indirect heater 2iof the thermistor 22 to produce an additional effect upon the resistanceof that thermistor. It will therefore be apparent that very slightchanges in the voltage applied by the system across l9 and 20 to theload will set into action correcting factors which will tend to holdthat potential very closely constant irrespective of changes in themagnitude of the load current.

It remains to consider the action of the shunt path in drawing the shuntcurrent IT which changes greatly for small changes in the voltageapplied to the load terminals. Assume that the system is in operationwith the relay 24 energized as shown in Fig. 5 so that its armature 3|is withdrawn from the fixed contact 30. The shunt path connecting pointsl9 and 20 comprises a thermistor 27, a special resistor 26, winding ofrelay 2d, variable resistor 23 and the heater 2! of ther- Thecharacteristics of the elements of this shunt path are illustrated inFig. 6, it being understood that the apparatus is designed for operationfor shunt currents IT extending The voltage current characteristic Er ofthermistor 21 is such that the potential difference across thethermistor falls rapidly and non-linearly from its magnitude at currentIo to the magnitude which it assumes at the larger current Im. In fact,the thermistor 21 considered alone is unstable since the greater itscurrent and internal heating becomes, the less its resistance and thegreater the current it will tend to accept. This large change in currentis conducive to efiective regulation in its efiect upon the resistanceof the series thermistor 22. However, the fall in potential across thethermistor 21 of the shunt path would by itself be very detrimental tomaintenance of a constant potential across the load terminals I9, 20. Toreduce this potential variation across the shunt path as a whole whileretaining the feature of large change in current characteristicof thethermistor 27 there are introduced resistance element 23 and theresistance of winding 26 of relay 25. These may be made substantiallylinear so that their aggregate potential drop as represented by ER willbe a straight line. It will be apparent that adding the potentials ER.and ET will yield a more nearly horizontal potential throughout therange of In to Im. However, since ET is far from linear the resultantobtained by adding ER and ET will have a corresponding non-linearity orcurvature. This may be much reduced and the total shunt path potentialmade more nearly horizontal with change in current by the introductionof a resistor 26 which preferably consists of some such material assilicon carbide which has a negative coefiicient of resistance withincreased current. The terminal potential E26 which such a negativecoefiicient resistance element will support rises with current but lessrapidly than does the current. The addition of the potential E28 tothose of ER and ET yields a nearly horizontal electromotive force E1.throughout the range of shunt currents from In to Im. Thus, it ispossible to take advantage of the wide variation in currentcharacteristic of thermistor 21 without permitting the potentialdifference across the shunt path connected between points l9 and to varymore than a slight amount.

In operation, the shunt current I'r stabilizes at some magnitude atwhich the three potentials ER, E26 and Er will add up to equal the loadvoltage Er. which is available after subtracting the series potentialdrops across the filter 1 and thermistor 22 from the voltage deliveredat the output terminals of the rectifier. If the system be designed andthe variable resistors adjusted to cause the apparatus to tend tooperate at a point such that the shunt current I'r lies between I0 andIm, as the voltage supplied across terminals l8 and 20 to the loadslightly decreases the current I'r will greatly increase. Conversely, ifthe load voltage Er. increases the current Ir will decrease.

The purpose of the third transformer winding 28, the relay 25 and thevariable resistor 29 is to provide, at the time the circuit isenergized, a high enough voltage to thermistor 21 to bring it within itsoperating range. When this operating range of temperatures has beenreached the 7 current through the thermistor 21 becomes great enough tooperate marginal relay 25 thus actuating its armature 3| to withdraw itfrom engagement with the contact point 36.

Fig. '7 discloses a system for rectifying alternating currents which issimilar to that of Fig. 5 in that it involves an L network comprising ashunt control arm to control indirect heating of a series thermistorarm. As shown the apparatus is in unenergized condition with itsstarting switches open and the various relays and control circuits inreleased or idle condition. In this circuit a source 32 may be connectedthrough switch 33 with current supply transformer 34 from the secondarywinding of which leads 35 and 36 extend to the full wave rectifier 31.Connected in series in the alternating current input lead 36 are ballastlamp 38 and a thermistor 39. The unidirectional output current leads 40and 4| extend from the rectifier by way of the smoothing choke 42 andthe charge fuses 43 and, the switch 44 to the terminals of a storagebattery 45, and from these terminals through the discharge fuses 46 tothe terminals ofthe unidirectional current load 41.

The ballast lamp 38 by abruptly changing its resistance to a highermagnitude when the safe rectifier current is exceeded preventsoverloading of the rectifier. The resistance of the thermistor 39 variesin accordance with the series load current therethrough. It is alsoindirectly affected by an associated heater element 49 which isintermittently operated in response to the operation of a shunt controlcircuit. The shunt control circuit leads from point 50 by way of theregulator fuses 5| to the four-arm Wheatstone bridge comprisingthermistor 52, fixed resistor 53, thermistor 54, variable tap resistorand resistor 56 to point 6| and by way of conductor 85 to point 51. Fromthe conjugate point 58 a variable tap 10 and lead 59 provide a path tojunction point 60 of a two-arm varistor valve. From the other conjugatepoint 62 of the shunt control bridge a lead 63 connects the winding oi.

relay 64 to' one terminal 65 of the valve while a path 66 connects thewinding of relay 6'! to the remaining terminal 68 of the varistor valve.If the variable tap 16 be properly positioned at a given temperature anda given applied load volta e between points 58 and 51 the shunt controlbridge will be balanced and no difference of potential will existbetween point 62 and 18. If the rectified voltage impressed betweenpoints 50 and 51 should rise the bridge will be unbalanced to sendunidirectional current over the circuit 59, 66 in one direction. If theimpressed voltage across points .50 and 51 should fall the bridge willbe unbalanced to send current over the circuit 59, 63, in the oppositedirection.

The two-arm varistor valve includes an asymmetrically conducting armadjacent the terminal 65 which is so poled as to permit current of onepolarity only to pass to the winding of relay 64. This, in effect, makesthe relay 64 a polarized relay. Similarly current of opposite polarityonly is permitted by the other asymmetric arm adjacent terminal 68 topass through the winding of relay 61, thus effectively making that relaypolarized but in an opposite manner from relay 64.

With the apparatus in proper adjustment, assuming that the switches 33and 44 have just been closed, the thermistors 52 and 54 of the shuntcontrol bridge will be cold and the shunt control bridge will beunbalanced in such direction as to send current from the storage battery45 through the winding of the starting relay 64. The relay 64 thereuponenergizes attracting its armature 1| and closing at its contact 12 acircuit extending through the heater windings 14 and 15 of thermistors52 and 54, respectively. In a few seconds thermistors 52 and 54 havewarmed to such a degree as to greatly reduce their resistance. Underthese circumstances current will traverse these thermistor elements 52and 54 in sufllcient magnitude to maintain their temperature within thedownward operating range of their resistance temperature characteristic.In consequence of the reduction of resistance of the thermistors 52 and54 the shunt control bridge becomes unbalanced in the opposite directioncausing the starting relay 64 to release and sending a current throughthe winding of relay 61. The release of relay 64 interrupts the heatingcurrent passing through heaters 14 and 15 of thermistors 52 and 54 butthe current which now passes directly through these thermistors issuflicient to maintain their operating temperature.

At the time that switch 44 is closed in starting the apparatus currentis applied through the fuses 5| over leads 85 and 86, fixed contact 16and armature 11 of relay 61 to operating winding 18 and resistancewinding 19 of relay 80. Relay thereupon energizes and attracts its outerarmature 8| to close the circuit through heater 49 of series thermistor39. At the same time it attracts inner armature 82, establishing aholding circuit for itself by an alternative path around armature 11 sothat a: momentary actuation of relay 61 to open the energizing circuitof relay 88 at contact 16, 11 will not cause relay 88 to deene'rgize. Itwill be apparent, therefore, that as soon after the switch 44 is closedconnecting storage battery 45 to the terminals of the control bridge andto the leads 85, 86 of the circuit of relay 80 as will afford suflicienttime for relay 80 to energize heating of the thermistor 39 will begin.It will continue until such a time as relay 61 is energized breaking thecontact between its armature 11 and the fixed contact 16 and movingarmature 11 into engagement with contact 84, thus short-circuitingwinding 18 and permitting the release of armature 8|.

In one embodiment of the circuit of Fig. 7 with a alternating currentsupply at the primary 7 winding of transformer 34? of 115 volts, 50 to60 cycles, the apparatus was designed for battery charging service tosupply a current varying from zero to one ampere at 17 volts. In thisparticular embodiment the resistance of element 53 was 700 ohms, element55 was 400 ohms, and

element 56 was 300 ohms. The heater windings l4 and 15 of thermistors 52and 54 were of 140 ohms resistance. Upon operation of relay 6 3 thesewindings were subjected to an alternating voltage of 24 volts. With thecontrol bridge balanced the potential difierence between the conjugatepoints 58 and 62 is of course zero. When, however, the circuit is firstput into operation by closing switches 33 and M, since thermistors 52and 54! are cold and their resistance is high, the bridge is very muchunbalanced so as to make the point ill highly positive with respect tothe point 62. When in a few seconds the resistance of thermistors 52 and5 3 has been reduced by the heating effect of heaters Hi and the bridgebecomes unbalanced in the opposite direction to make terminal 52positive with reference to terminal 58. Thereafter in operation theterminal 62 always remains positive with reference to terminal 58 sothat current passes at all times through the winding of relay Bl. Relay6! is marginal in its operation and is sensitive enough to operate on anunbalanced voltage such as is impressed upon it when the unidirectionalvoltage across points and 51 rises to 17.3 volts and releases when thatimpressed electromotive force falls to 17.0 volts. Accordingly, so longas the circuit remains in operation the thermistor 39 will begin toreceive heat from its heater Q9 shortly after the load voltage falls to17.0 volts and will continue to receive heat until the load voltagerises to 1743 volts. The output voltage does not rapidly change betweenthe limiting values of 17.0 and 173 volts since the thermistor 39 andits heating element have a considerable thermal inertia. Accordingly, atthe beginning of the heating operation heat is applied slowly and at theend of the heating operation heat is applied more slowly than in themiddle of the heating period. Moreover, after the heating current hasbeen interrupted heat continues to be applied to the thermistor indecreasing degree. This tends to greatly smooth the regulatingoperation.

What is claimed is:

1. In combination a rectifier having input terminals to which a sourceof alternating voltage may be connected and output terminals, means forregulating the voltage of the rectified current delivered from theoutput terminals of the rectifier to a pair of load terminals includinga thermistor connected in series between one of the output terminals andone of the load terminals to set up a potential difierence between theterminals which varies inversely with the load current traversing thethermistor, and means energized by an external source to heat thethermistor to a sensitive point on its resistancetemperaturecharacteristic.

2. A transformer having a primary winding on which an alternatingvoltage to be rectified may be impressed, a secondary windinginductively associated with the primary winding, a full wave rectifierhaving input terminals connected to the secondary winding and havingoutput terminals adapted to be connected to a load circuit through aregulating system, said regulating system comprising a thermistor and afilter connected in tandem relationship to the output terminals of therectifier and a second secondary winding associated with the primarywinding and connected to the thermistor to supply heating energythereto.

3. A regulating system for controlling the Voltage of the energysupplied to a load comprising an alternating current transformer havinginput terminals and output terminals, a source of uninterruptedalternating current, a conductiv circuit continuously connectingterminals of the alternating current source to the input terminals ofthe transformer irrespective of load conditions, a load, a load circuitconnecting the output terminals of the transformer to the terminals ofthe load and including a series variable resistance thermistor throughwhich the load energy passes, said thermistor and said transformerwindings being in intimate heat conducting relationship whereby the heatgenerated in the transformer 29 may control the resistance of thethermistor to vary the voltage supplied to the load terminals.

4. A regulating system for controlling the voltage of the-energysupplied from a source of voltage to a load comprising a translatingdevice having input terminals, a conductive circuit for continuouslyconnecting the terminals to a source of alternating current irrespectiveof conditions of the load, said translating device also having an outputcircuit including terminals for connection to the load, said circuitbeing continuously closed, and said translating device operating tomodify the form of the energy transferred from the source to facilitateits utilization by the load, said translating device presentingresistance whereby heat is generated by the translating device, athermistor connected in series relationship in a continuously energizedcircuit to one of the terminals of the translating device so that energypassed to the load traverses the thermistor whereby the thermistor mayimpose a varyin loss of potential to regulate the load voltage as theenergy supplied to the load varies, the translating device and thethermistor being mounted in such intimate heat transfer relationshipthat heat gen- 45 erated by the translatin device afiects thetemperature of the thermistor and hence its resistance.

5. In combination, a transformer having a primary winding, mainsecondary winding induc- 0 tively associated with the primary winding,an

auxiliary secondary winding also inductively associated with the primarywinding, a full wave rectifier having input terminals connected to themain secondary winding and output terminals, a thermistor and a filterconnected in tandem to the output terminals of the rectifier to regulatethe voltage supplied to the load and to reduce ripples, and a heatingcircuit for the thermistor including the thermistor and the auxiliarysecondary winding.

6. A source of current, a load, a circuit connecting the source to theload, a shunt path across the connecting circuit including a thermistorwhereby the shunt path diverts currentfrom the load to regulate theterminal voltage applied to the load, means for initially heating thethermistor to a temperature at which its resistance is suitable forregulating purposes and means responsive to current passing through theshunt path for disconnecting the initial heating means when the currentin the shunt path attains a predetermined magnitude.

'7. A source of voltage, a load, means comprising a series thermistorhaving a falling resistance temperature characteristic throughout adesired range of temperatures connecting the source to the load toregulate the voltage supplied to the load in accordance with the currentsupplied to the load, an external heater for the thermistor tosupplement the heat supplied by the series load current passingtherethrough, a path connected in shunt to the load and having twopoints which are at the same potential when the total outputelectromotive force across the shunt path is at a normal desired value,a relay having a winding connected between said two points and having anarmature and a, contact therefor, and a circuit connected to said sourceto derive an electromotive force therefrom and including in series saidexternal heater and said armature and contact.

8. In combination, arectifier having input terminals to which a sourceof alternating voltage may be connected and output terminals, means forregulating the voltage of the rectified current delivered from theoutput terminals of the rectifier to a pair of load terminals includinga thermistor having a falling resistance temperature characteristicconnected in series between one of the output terminals and one of theload terminals to set up a potential difierence between the terminalswhich varies inversely with the load current traversing the thermistor,and means energized by an external source to heat the thermistor to asensitive point in the temperature zone in which the thermistor exhibitsthe falling resistance-temperature characteristic.

9. In combination, a source of alternating current, a full waverectifier, a regulating thermistor and a load, all in series circuitrelation whereby said source supplies unidirectional current energy tothe load, a shunt circuit including a thermistor connected across saidload and means controlled by the, thermistor of the shunt path to causeabrupt variations in the temperature of the thermistor in the seriescircuit.

10. In combination, a source of electrical energy, a series impedor anda load, all connected in series circuit, a Wheatstone bridge pathconnected in shunt to the load and having two points conjugate to theconnection points so that for a predetermined terminal voltage nodifierence in potential exists between the conjugate points, twoasymmetrically conducting paths connected between the conjugate pointsand each including an individual relay, means whereby operation of onerelay in response to unbalance potentials serves to initiate a reversalof the direction of unbalance of the bridge and means whereby operationof the other relay in response to oppositely directed unbalancepotentials serves to regulate the impedance of the series impedor tocontrol the voltage impressed upon the load.

JAMES A. POTTER.

